The oil and gas industries utilise a technology known as hydraulic fracturing or “fracking”. This normally involves the pressurisation with water of a system of boreholes in oil and/or gas bearing rocks in order to fracture the rocks to release the oil and/or gas.
In order to achieve this pressurisation, valves may be used to block off or isolate different sections of a borehole system. These valves are referred to as downhole valves, the word downhole being used in the context of the disclosure to refer to an article that is used in a well or borehole.
Downhole plugs are one type of valve. A conventional plug consists of a number of segments that are forced apart by a conical part. The cone forces the segments out until they engage with the pipe bore. The plug is then sealed by a small ball. Another way of forming such valves involves the use of spheres (commonly known as fracking balls) of multiple diameters that engage on pre-positioned seats in the pipe lining. Downhole plugs and fracking balls may be made from aluminium, magnesium, polymers or composites.
A problem with both types of valve relates to the ductility of the material used to make them. Corrodible magnesium alloys such as those used to make downhole valves have limited ductility due to their hexagonal crystal structure. These alloys can exhibit significant crystallographic texture (ie crystals aligned in a particular direction) when used in their wrought form, such as when they are extruded. This can further limit ductility, especially in the transverse direction. These factors mean that the ductility of dissolvable magnesium alloys is lower than is desirable.
The applicant's earlier patent application, GB2529062A, relates to a magnesium alloy suitable for use as a corrodible downhole article. This document discloses an alloy comprising 3.7-4.3 wt % Y, 0.2-1.0 wt % Zr, 2.0-2.5 wt % Nd and 0.3-1.0 wt % rare earths having a maximum elongation (ie ductility) of 21%, a corrosion rate of around 1100 mg/cm2/day in 3% KCl at 93° C. (200 F) and a 0.2% proof stress of around 200 MPa. The range of uses of these magnesium alloys can be limited by their ductility.
CN 106086559 describes magnesium alloys comprising Gd and/or Y as well as Ni. However, the atomic percentage amounts of Y and/or Gd in these alloys correspond to weight percentages which are greater than 2 wt % Y and/or greater than 7 wt % Gd. CN 104152775 relates to a magnesium alloy comprising 86.7 wt % Mg, 2.2 wt % Ni, 5.8 wt % Gd and 5.3% Nd.
A material which provides the desired corrosion characteristics, but with improved ductility, has been sought.